Small electric motors have been made in two different types: a first type is two individual bearings supported at opposite ends of the frame to support and journal the rotor-mounted shaft; and a second type is a unit bearing motor wherein the frame carries a post at only one end which has an elongated unit bearing within to journal the shaft. These first and second types are shown in FIGS. 11 and 2, respectively, of U.S. Pat. No. 3,874,073. This patent utilized a frame made of resin and a particulate matter such as sand, with the frame supporting the magnetic stator of the motor. In unit-bearing motors, there was the difficulty of achieving uniform air gap between the stator and rotor, and U.S. Pat. No. 3,755,889 showed the use of shims to attempt to achieve this uniform air gap.
Another way to attempt to achieve a substantially uniform air gap was to utilize a cast or die-cast metal frame of iron or aluminum, with the frame supporting the magnetic stator and also having unitary therewith a central apertured post which had the unit bearing therein. This structure is shown in a number of patents, such as U.S. Pat. Nos. 2,522,985; 4,045,698; 4,209,722; and 4,499,661. U.S. Pat. No. 3,293,729 disclosed a combined frame and stator yoke made from a plurality of laminations so that it would carry magnetic flux, and this frame had welded thereto a unit bearing post to journal the rotor. U.S. Pat. No. 4,565,937 disclosed a unit-bearing motor with a cast aluminum frame and then an insert bushing of a Babbitt sleeve or cast iron sleeve to provide the unit bearing for the rotor.
Where the unit-bearing frame is made from cast aluminum, an aluminum post results which has journal bearing qualities inferior to cast iron. The patentees of U.S. Pat. No. 4,565,937 recognized this and provided an insert of a superior bearing material. However, this has the disadvantage of increased assembly problems, and with two dissimilar metals there are two different thermal expansion characteristics to increase the problems of attempting to obtain a substantially uniform air gap. Where the unitary frame and post are made from cast iron, this results in the disadvantage of being a large casting which must be machined at the inner stator mounting area to be able to receive the stator laminations and then be machined at the journal bearing aperture. In the prior art, these two machined portions have been machined with the same chucking in order to achieve the best concentricity. However, this means that the machining of the journal bearing aperture is accomplished at a low speed to accommodate the proper surface cutting speed at the stator mounting area. Also, such castings, whether aluminum or cast iron, are relatively expensive because they are larger than the stator diameter and require machining in order to be able to be used.